The study, in the The American Journal of Obstetrics and Gynecology, finds that looking at five proteins in a woman's blood early in pregnancy can help indicate her risk of delivering prematurely toward the end of pregnancy.

There's not much that can be done to prevent premature birth at this point, but lead author Dr. Thomas McElrath says it's still helpful to know if a woman is at high risk, and the blood test could eventually be used to figure out preventive treatments depending on each expectant mother's profile.

"At the end of the day," he says, "the objective would be to try to have a highly personalized, almost a fingerprint or pattern for each pregnancy, and to suggest: These are the therapies that might be most applicable."

For now, the blood test needs to be studied more before it can be used in the clinic. Other efforts to develop tests early in pregnancy for risk of preterm birth are also under way.

A typical first-time mother's risk of spontaneous delivery before 35 weeks is about 5 percent; the blood test could suggest that risk is as high as 20 percent or as low as 2 percent, the paper reports. The test measures "circulating microparticle proteins" that can travel from one cell to another and, the paper says. They may offer "a new window" into early pregnancy — including slight imbalances in physiology that, with time, may "reverberate and amplify," leading to harm toward the end of pregnancy, the paper says.

"We're beginning to appreciate that, to an increasingly large degree, how the placenta is formed at the end of the first trimester, and how it interacts with the maternal system, may actually predispose women to several types of adverse pregnancy outcome," Dr. McElrath says.

That's long been thought about preeclampsia, he adds, "but what we're being to see is that a large portion of pregnancies that end up with preterm labor or preterm rupture of membranes may also have some abnormal placental interaction at the beginning of the pregnancy."

There can be other reasons for preterm birth, including infections in late pregnancy, McElrath says. But insight into these earlier potential factors could lead both to better risk assessment and, as the biology becomes more clear, to treatments.